Such turbomachines are known in the prior art in the widest variety of developments. A pressure, which lies below or above the atmospheric pressure of the turbine environment, prevails inside the machine housing, depending upon the type of turbomachine. A basic problem with turbomachines is that the annular gap which exists between the shaft and the machine housing cannot be completely sealed on account of the relative movement between the shaft surface and the adjacent machine-housing surfaces. This leads to environmental fluid being able to penetrate into the interior of the machine housing, for example in the case of steam turbines with exhaust steam pressures below atmospheric pressure, which leads to an adverse effect in the operation and efficiency of the steam turbine. In the case of compressors or gas turbines, in which the machine-housing internal pressure is higher than the machine-housing ambient pressure, the process fluid, however, can escape from the interior of the machine housing into the machine-housing environment, which has a negative effect upon the thermal efficiency of the turbomachine.
A turbomachine in the form of a steam turbine with a conventional shaft seal arrangement is subsequently described with reference to FIGS. 1, 2 and 3, wherein FIG. 1 shows a schematic view of the steam turbine, FIG. 2 is a schematic view of a rear seal shell of the machine housing of the steam turbine which is shown in FIG. 1 and has a conventional shaft seal arrangement, and FIG. 3 schematically shows the pressures which prevail in the shaft seal arrangement which is shown in FIG. 2.
The steam turbine 100 comprises a machine housing 102 through which extends a shaft 104 which is guided out of the machine housing 102 on both sides. The machine housing 102 is provided with a front seal shell 106 and a rear seal shell 108, wherein the rear seal shell 108 presently undertakes the task of sealing the pressure chamber 110, which is defined inside the machine housing 102, in relation to the machine-housing environment. Live steam is fed to the pressure chamber 110 of the steam turbine 100 via a feed line 112 which is provided with an emergency stop valve 114 and a control valve 116 which are in series in the flow direction. After expansion of the steam in the steam turbine 100, the exhaust steam, via an exhaust line 118, is fed to a condensing plant 124, which is provided with a cooling device 120 and is functionally connected to an evacuation device 122, and is condensed there.
The resulting condensate is conducted out of the condensing plant 124 via a line 126.
For sealing the annular gap 125 which exists between the shaft 104 and the rear seal shell 108, the rear seal shell 108, which is installed in the machine housing 102 in a fixed manner and with sealing effect or together with the machine housing 102 forms a constructional unit, is provided with a shaft seal arrangement 128, as is shown in FIG. 2. Starting from the pressure chamber 110, the shaft seal arrangement 128 comprises at least three consecutive sealing modules, specifically an inner labyrinth seal 130, a middle labyrinth seal 132 and an outer labyrinth seal 134, which seal the annular gap 125.
During operation of the steam turbine 100, exhaust steam, under an exhaust steam pressure pAD, is present in the rear exhaust steam region 138 of the machine housing 102. Air under an ambient pressure pU, which lies above the exhaust steam pressure pAD, is present outside the machine housing 102. From the exhaust steam region 138, seal steam under the seal steam pressure pSD is introduced into the annular gap 125, via a seal steam feed line 140, between the inner labyrinth seal 130 and the middle labyrinth seal 132, the seal steam pressure pSD lying slightly above the ambient pressure PU. In this case, the seal steam pressure pSD and also the temperature of the seal steam must be accurately regulated or adjusted in order to avoid leakages or damage as a result of overheating or condensate on the rear seal shell 108 and on the shaft 104. The pressure gradient of the seal steam reduces in the direction of the exhaust steam region 138 across the inner labyrinth seal 130 to the pressure level of the exhaust steam pressure PAD and in the direction of the environment reduces across the middle labyrinth seal 132 to the leak-off steam pressure pWD which prevails between the middle labyrinth seal 132 and the outer labyrinth seal 134, wherein the leak-off steam pressure pWD, contingent upon the exhaust stack draft in the leak-off steam discharge line 142, lies slightly below the machine-housing ambient pressure pU. Depending upon the level of the seal steam pressure pSD which is supplied via the seal steam feed line 140, it is also conceivable that the seal steam pressure pSD, for constructional reasons, does not fully reduce across the middle labyrinth seal 132 to the level of the ambient pressure pU and steam escapes to the outside via the outer labyrinth seal 134. In order to prevent this, further labyrinth seals, with leak-off steam discharge lines in between, can be optionally connected downstream, which is not shown in FIG. 2, however. Alternatively, a condensing plant (not shown either) can be connected downstream to the leak-off steam discharge line 142 for supporting the suction action. Depending upon the accumulating amount of condensate, condensate drains 144 can be provided between the individual labyrinth seals 130, 132, 134, wherein in FIG. 2 only one condensate drain 144 between the middle labyrinth seal 132 and the outer labyrinth seal 134 is shown.
One problem with the arrangement shown in FIG. 2 is that the use of seal steam as seal fluid results in a very complex and expensive construction, which is to be attributed in particular to the complex controlling of the seal steam and to the provision of the leak-off steam discharge line.
A further turbomachine is disclosed in DE-A-36 12 327.